Integrated Services Digital Network (ISDN) is the name given to the combined digital telephony and data-transport services offered by telephone carriers. ISDN involves the digitization of the telephone network, which permits voice, data, text, graphics, music, video, and other source material to be transmitted over existing telephone wires. The emergence of ISDN represents an effort to standardize subscriber services, user/network interfaces, and network and internetwork capabilities. ISDN applications include high-speed image applications (such as Group IV fax), additional telephone lines in homes to serve the telecommuting industry, high-speed file transfer, and videoconferencing. Voice service is also an application for ISDN.

Some of the advantages of ISDN are:

• Dialup is fast - ISDN calls typically dial and connect in 1 to 3 seconds.
  
  
• It's digital - 64 kbps bandwidth for each "B" channel is guaranteed.
  
  
• It's multi-mode - A "B" channel can carry data, voice, fax or video.
  
  
• It concentrates calls - a PRI connection can deliver 30 concurrent calls through one cable. A BRI delivers 2 calls through one cable.
  
  
• You can associate many telephone numbers to the same line (at a fraction of the cost of separate, multiple telephone lines).